In the oil and gas industry, magnetic azimuth measurements referenced to the Earth's magnetic field in downhole tools require extremely sensitive magnetic sensors, such as magnetometers. These measurements often need to be as accurate to a few nanotesla to obtain accurate measurements and thereby allow a well operator to ascertain the correct well direction of an associated downhole tool. However, there are many sources of error for measurements obtained from magnetic sensors, and it is critical to minimize each of them in order to maximize the accuracy of the measured azimuth.
In some cases, for instance, time dependent sources may cause error in magnetic sensor measurements. Time dependent sources for error include drift in scale and bias of the magnetic sensor and electronics due to thermal aging and mechanical shock. Another time dependent source for error is change in the magnetization of nearby magnetically permeable materials as well as magnetic fields generated by current flowing in conductors that run adjacent or nearby the magnetic sensor.
There are many reasons why current must flow past sensitive magnetic sensors in downhole tools. For example, power sources such as batteries or generators and electrical loads (e.g., electronics, sensors, actuators) can be positioned in a variety of locations along a tool string or in a downhole tool. This necessitates that signals and power be transmitted through and within individual downhole tools. Current provided by the power sources sometimes runs adjacent to the magnetic sensors, which are susceptible to magnetic interference caused by these currents.